1. Field of the Invention
The present invention relates to a semiconductor laser module that is constituted by optically coupling an optical fiber having a fiber grating and a laser diode.
2. Description of the Related Art
FIG. 19 is a drawing showing the configuration of a semiconductor laser module 600 shown in xe2x80x9cElectronics Lettersxe2x80x9d Vol. 23, No. 7 published on Mar. 26, 1987. The semiconductor laser module 600 comprises a laser diode (LD) 1, and an optical fiber 2 in which a fiber grating (FG) is formed. The FG is formed in an optical fiber by ultraviolet exposure so that the refractive index of the optical fiber can be periodically changed, and only light having a specific wavelength and of a fixed amount can be reflected, according to the period of change in this refractive index. FIG. 20 is a drawing described in U.S. Pat. No. 4,725,110 showing an appearance when the FG is formed. An FG 5 is formed in an optical fiber 4 using ultraviolet (UV) light from a UV irradiator 3.
FIG. 21 is a drawing showing an optical spectrum (a solid line 6) of an LD when no FG is formed in an optical fiber and an optical spectrum (a solid line 7) of an LD when an FG having the reflection center wavelength of 980 [nm] is formed in the optical fiber. Although the oscillation center wavelength of the LD when no FG is provided is 982 [nm], the oscillation center wavelength when the FG is provided is set to 980 [nm] which is the reflection center wavelength of the FG, and hence it is known that the oscillation center wavelength of the LD is controlled with the FG. Further, if the oscillation center wavelength of the LD when no FG is provided in an optical fiber and the reflection center wavelength of the FG are made to approach each other to some extent, the wavelength characteristics of the light output by the optical fiber can be stabilized. If the gain center wavelength of an LD and the reflection center wavelength of the FG are made to approach to some extent, the wavelength characteristics of light output by the optical fiber can be stabilized.
In a conventional module that returns the light having a specific wavelength to an LD with an FG, when a monitor photodiode that monitors the backlight of the LD at the rear side of the LD is provided, the optical output of an optical fiber versus the current of the monitor photodiode (monitor current) becomes unstable in the low optical output region (in the vicinity of 10 to 40 mW). On the other hand, the optical output versus the operating current of the LD is stable even in the low output region. That is, the FG formed in the optical fiber changes the amount of light of the backlight of the LD and affects the monitor current versus optical output characteristics. In a conventional optical semiconductor module, to keep the optical output of the optical fiber constant, an APC (optical output constant control) circuit may be used in some cases that controls the drive of the LD so that the monitor current can be kept constant, thus keeping the optical output constant. However, it is difficult to control the optical output in the APC circuit if the monitor current versus optical output characteristic becomes unstable.
Further, in such module, a phenomenon called a kink occurs in which the optical output of an optical fiber end does not increase even if the operating current of an LD is increased. Such kinks easily occur due to axis misalignment in the X direction or Y direction of the optical fiber. It is important to suppress the generation of kinks caused by optical axis misalignment of the optical fiber in a semiconductor laser module that requires a high optical output.
One object of the present invention is to provide a semiconductor laser module in which the monitor current versus optical output characteristics are stabilized. A further object of the present invention is to suppress the generation of kinks caused by optical axis misalignment.
A first semiconductor laser module of the present invention comprises a laser diode that emits light, an optical fiber that transmits the light emitted by the laser diode to the front side, a fiber grating which is formed in the optical fiber and reflects the light in a specific wavelength region mainly having a longer center wavelength than the oscillation center wavelength or gain center wavelength of the laser diode to the laser diode side, and a monitor photodiode that receives the light emitted by the laser diode to the rear side. Thus the monitor current versus optical output characteristics can be stabilized.
A second semiconductor laser module of the present invention comprises a laser diode that emits light, an optical fiber that transmits the light emitted by the laser diode to the front side, a fiber grating having a reflection spectrum half-width of 0.8 nm and over which is formed in the optical fiber and reflects the light in a specific wavelength region, and a monitor photodiode that receives the light emitted by the laser diode to the rear side. Thus, the monitor current versus optical output characteristics can be stabilized.
In the first or second semiconductor laser of the present invention, the optical fiber may also comprise a wedge-shaped tip that functions as a lens arranged so that magnification will become higher than the magnification at which the coupling efficiency between the laser diode and the optical fiber is maximized. Thus, the generation of kinks caused by optical axis misalignment of an optical fiber can be suppressed.